LOL just making you aware of what lays ahead my friend. I too wanted to do this but after what was pointed out in regards to water, I gave up. Yeh you can use the sculpt tool as dave suggested. It depends on how much power your PC has too, I prefer to make mine in PS with clouds difference or similar. I can get a landscape out in 5 mins or less now.

Scale etc arent really that hard. First determine how big you actually want the scene. TBH if its an animation this will be big, if its just for render shots it doesnt have to be big at all, again depends on what you want. The ones I make in PS and use displacement usually end up a few kilometres across, depending on the effect. Sometimes I use both methods together, a displace from PS then sculpt in Maya or Zbrush to add details etc etc.

I will be watching If you have trouble with scale etc just holler, Im not a 3D modellers bumhole but I am a good Surveyor .

Cheers bullet